This invention relates to a method for preparing a smooth electroding ink containing silver alloy powder particles which ink is suitable for forming the internal electrode films of a monolithic ceramic capacitor.
Only in recent years have silver containing inks been used in the demanding role of precursors of the buried electrodes in monolithic ceramic capacitors. Such glass-ceramic capacitors are described in U.S. Pat. No. 3,682,766 issued Aug. 8, 1972 and assigned to the same assignee as the present invention.
When in a monolithic ceramic capacitor the roughness of an internal electrode film is commensurate with the thickness of an adjacent active dielectric layer, the capacitor tends to exhibit a low breakdown voltage and poor performance on life tests. More generally, in such capacitors with rough electrodes, the maximum volt-microfarad product of which it is capable is less than that possible based upon the inherent dielectric properties of the ceramic and the use of smooth electrodes. The thinner are the active dielectric layers, the smoother the electrodes must be for optimum capacitor performance.
Suitably smooth electroding inks containing silver are much more difficult to prepare than the more commonly used inks based upon other base or noble metals.
Fine silver powders may be produced either by a chemical or a ball milling method. The resulting fine metal powder is then removed from the chemical or milling liquid, dried, and dispersed in a suitable vehicle for forming an electroding ink. This dispersing step is usually carried out in a 3-roll mill.
It is further recognized that two competing processes normally occur during the milling of malleable metal particles, namely comminution (or deagglomeration) and welding. Silver containing metal surfaces will readily weld due to compaction of particles by the milling elements (e.g. balls). This leads to the creation of agglomerate particles tending to increase the average particle size of the very malleable and ductile silver powder.
Fine malleable base-metal flake is known to be produced by milling in a sand mill, which flake is particularly useful as a bright pigment in decorative paints. (Fine powders of the same metals, wherein the particles are spherical appear very dark or black). Sand mills employ small milling balls, for instance from 0.2 to 0.02 inch (0.5 to 0.05 cm) in diameter (equivalent to a volume less than about 0.07 ml.) while ball milling may be distinguished by typically employing milling balls having volumes greater than 0.07 ml. Sand milling is accomplished by means of a rotating impeller and normally involves a more vigorous agitation, compared to the relatively slow tumbling of the large balls in a ball milling method. Certain sand mills, having multiple or compound impellers designed for high speed comminution, are known as ATTRITOR mills (Trade Mark of Union Process Company). In the production of fine spherical malleable metal particles containing silver, sand milling has not heretofore been employed for deagglomerating silver containing powders since it is known that for the milling times required for effective deagglomeration, flake would be produced.
In monolithic capacitor manufacturing processes for building the alternate layers of green ceramic and electrode ink films to form a stack, essentially spherical metal particles are greatly preferred in the ink as opposed to flake e.g. to avoid trapping the electroding ink vehicle within a dense network of flakes during ink film drying and firing steps.
Known procedures for making electroding inks for use in monolithic ceramic capacitors include separating the finely ground metal from the milling vehicle and then drying and dispersing the comminuted powder in another vehicle suitable for making the electroding paste. Many factors come to bear upon the choice of the electroding ink vehicle. The electroding ink should possess the required rheological properties for screen printing. The chemical properties must insure compatability with the green and mature ceramic at high temperatures at which the stack of alternate ceramic and electrode films is fired. The absence of large particle agglomerates is essential for providing smooth electrode layers in monolithic capacitors. There are no known liquids that are effective as a ball or sand milling vehicle and that at the same time also lead directly to an electroding ink having the many above-noted desired qualities.
It is therefore an object of the present invention to provide a smooth silver-containing electroding ink by a process wherein one liquid is employed both as the comminuting vehicle and the ink vehicle.
It is also an object of this invention to provide a simplified process for making large quantities of an electroding ink wherein the conductive particles are essentially spherical silver alloy particles.
It is still a further object of this invention to provide a silver containing electroding ink suitable for making monolithic ceramic capacitors having thin active dielectric layers down to thicknesses less than 0.001 inch (0.0025 cm).